Nowadays, more and more systems are transmitting video data, using different types of transmission media. Inter alia, many videoconference systems are being put on the market. The simplest of these systems are designed for use between personal computers (PC) connected to a local area network (LAN). All these systems have very different features, quality, and of course prices. There is no easy way objectively to compare these different systems.
FIG. 1 is a schematic view of the basic structure of a videoconference system, in which the invention may be implemented. The videoconference system of FIG. 1 comprises two PCs 1 and 2, linked through a LAN 3; each PC is provided with a microphone 5, 6, a headset 7, 8 and a camera 9, 10, and a display 11, 12. The video image and audio information is transmitted from one PC to the other, so as to permit video conferencing between the PC users.
The following problems are still encountered in new video transmission systems such as videoconference systems:
the image is not as fluid as in TV programs; all movements do not appear on the screen, and some images are lost; PA1 during communication, there are some interruptions, or `holes` in the sound; PA1 the audio and image streams are not always synchronised, so that lips movements do not exactly correspond to the words heard; PA1 there are transmission delays, and it may sometimes be necessary to wait until a correspondent has listened to a previous utterance before being able to speak. PA1 image frame rate; PA1 number of lost frames; PA1 number and duration of audio interruptions, that is of so-called audio `holes`; PA1 image and sound synchronisation; PA1 transmission delay. PA1 (a) providing a reference code in at least some of the video frames of the transmission transmitted from the source, the reference code taking the form of a predetermined pattern of areas in the image represented by the video signal with each said area being capable of assuming any one of a respective plurality of color states, each combination of color states assumed by the areas constituting a code value for the reference code; PA1 (b) causing the reference code provided in the transmitted video frames to evolve over time through a predetermined sequence of code values; PA1 (c) receiving the transmission at the destination, detecting the reference code in the received video frames and determining the code values of the reference code; and PA1 (d) carrying out quality measurements on the coherence of transmission as received at said destination using the codes values of the detected reference codes as a reference indicative of the transmission as sent from the source. PA1 means for receiving the transmission at the destination; PA1 means for detecting the reference code in the video frames received at the destination and determining the code values of the reference code; and PA1 means for carrying out quality measurements on the coherence of the transmission received at the destination using the codes values of the detected reference codes as a reference indicative of the transmission as sent from said source.
These problems may be associated with the following parameters of the video transmission system: